In general, an electrophotography in a PPC (Plain Paper Copy) copier or a printer for transferring a toner image formed on a photo-sensitive material to recording paper is carried out in the procedure described below. First, an electrostatic latent image is formed on the photo-sensitive material, the latent image is developed by using a toner, the toner image is transferred onto a sheet to be fixed such as paper or the like, and then the transferred toner image is fixed by heating with a heat roll or a film. Since the fixation is carried out under heating in a state that the heat roll or the film is directly brought into contact with the toner on the sheet to be fixed according to this method, it is performed in a short period of time and with a very high thermal efficiency, thereby achieving a very good fixing efficiency.
However, though having a good thermal efficiency, the heat fixing method has a problem of a so-called offset phenomenon since the toner is brought into contact with the surface of the heat roll or the film in the melt state of the toner. With fast printing speed, so-called low temperature fixing performance for fixing at lower temperatures has been in demand for a toner.
In order to obtain a resin excellent in fixing properties and offset resistance, there has been known a resin obtained by using a high molecular weight resin and a low molecular weight resin in mixture and crosslinking a high molecular weight portion (for example, Patent Documents 1 and 2). However, sufficient low temperature fixing performance could not be obtained from these resins. Meanwhile, for the purpose of improving low temperature fixing properties, there has been disclosed a resin obtained by the addition of a different kind of resin such as crystalline polyester or the like to a base resin such as a styrene acrylic resin or the like (for example, Patent Documents 3 and 4). However, only with the addition, dispersibility of the crystalline polyester was low, and crystalline polyester was easily detached when it was used for a toner. With the addition of crystalline polyester, low temperature fixing properties were enhanced, but blocking was easily caused by the detached crystalline polyester, so that satisfactory storage stability has not been achieved.
There has been disclosed a resin in which a crystalline polyester resin was added to a base resin and both resins were chemically combined (for example, Patent Documents 5 to 7). However, since both resins were chemically combined, the crystalline polyester resin was melted in the base resin, the base resin was plasticized, and crystallinity of the crystalline polyester resin was reduced, so that satisfactory storage stability has not been achieved.
Meanwhile, the addition of a non-crystalline resin, an inorganic fine particle or an organic metal salt to a crystalline polyester resin has been disclosed in various documents (for example, Patent Documents 8 and 9). According to these documents, crystallinity was controlled with the addition of an inorganic fine particle or an organic metal salt, but it was difficult to increase the degree of crystallinity of the crystalline polyester when the compatibility between the crystalline polyester resin and the non-crystalline rosin was not controlled. As a result, satisfactory storage stability has not been achieved.
Meanwhile, in paragraph 0076 of Patent Document 10, when the toner of the present invention is produced according to a suspension polymerization method (to be described later), non-crystalline polyester is easily unevenly distributed on the surface of suspension droplets formed in an aqueous dispersion medium and as a result, non-crystalline polyester is unevenly distributed on the surface of toner particles of the toner to be produced. On the other hand, according to the document, crystalline polyester is unevenly distributed inside toner particles. Namely, according to the document, in the suspension polymerization method, crystalline polyester is present inside the toner, and non-crystalline polyester is present on the toner surface, so that it is found that the surface layer of the crystalline polyester is not coated with non-crystalline polyester.